Manufactured tin(II) sulfate granules for electrolytic coloring with metal salts

ABSTRACT

A manufactured stannous sulfate granulate is suitable for electrolytically coloring anodized aluminum with metallic salts, where the granulate particles have a linear extension from 0.1 to 10 mm. Also disclosed are a process for manufacturing the same and using it for electrolytically coloring anodized aluminum with metallic salts. The manufactured stannous sulfate granulate is characterized by technical advantages, such as storage stability, easy dispensibility, and absence of dust, compared to conventionally used stannous sulfate powders.

FIELD OF THE INVENTION

This invention relates to manufactured tin(II) sulfate granules for theelectrolytic coloring of anodized aluminum with metal salts, to aprocess for their production and to their use for the electrolyticcoloring of anodized aluminum with metal salts.

STATEMENT OF RELATED ART

It is known that, on account of its base character, aluminum becomescovered with a natural oxide coating generally below 0.1 μm in thickness(Wernick, Pinner, Sheasby, The Surface Treatment and Finishing ofAluminum and its Alloys, ASM International).

Considerably thicker oxide coatings can be obtained by electrolyticoxidation of aluminum. This process is known as anodizing. Sulfuricacid, chromic acid or phosphoric acid is preferably used as theelectrolyte. Organic acids, such as for example oxalic acid, maleicacid, phthalic acid, salicylic acid, sulfosalicylic acid, sulfophthalicacid, tartaric acid or citric acid, are also used in some processes.

However, sulfuric acid is the most commonly used electrolyte. Dependingon the anodizing conditions, layer thicknesses of up to 150 μm can beobtained in this process. However, layer thicknesses of 20 to 25 μm aresufficient for external applications, such as for example facade facingsor window frames.

The anodizing process is generally carried out in 10 to 20% sulfuricacid with a current density of 1.5 A/dm², at a temperature of 18° to 22°C. and over a period of 15 to 60 minutes, depending on the requiredlayer thickness and the particular application.

The oxide coatings thus produced have a high absorption capacity for anumber of organic and inorganic dyes.

Electrolytic coloring processes, in which anodized aluminum is coloredby treatment with alternating current in heavy metal salt solutions,have been known since the middle of the thirties. The heavy metals usedare, above all, elements of the first transition series, such as Cr, Mn,Fe, Co, Ni, Cu and, in particular, Sn. The heavy metal salts aregenerally sulfates, pH being adjusted to a value of 0.1 to 2.0 withsulfuric acid. The coloring process is carried out at a voltage ofaround 10 to 25 V and the resulting current density. Thecounter-electrode may either consist of graphite or stainless steel orof the same material which is dissolved in the electrolyte.

In this process, the heavy metal pigment is deposited in the pores ofthe anodic oxide coating in the half cycle of the alternating current inwhich aluminum is the cathode, the aluminum oxide coating being furtherthickened by anodic oxidation in the second half cycle. The heavy metalis deposited at the bottom of the pores and thus colors the oxidecoating.

Tin salts in particular are used, colors varying from champagne viavarious bronze tones to black being obtained according to the procedureadopted.

However, one of the problems encountered where coloring is carried outin tin electrolytes is that the tin readily oxidizes so that basictin(IV) oxide hydrates (stannic acid) are rapidly precipitated duringthe application and, in some cases, even during the storage of the Snsolutions. It is known that aqueous tin(II) sulfate solutions areoxidized to tin(IV) compounds simply by exposure to atmospheric oxygen.This is highly undesirable in the coloring of anodized aluminum in tinelectrolytes because, on the one hand, it disrupts the process sequence(frequent renewal or topping up of the solutions rendered unusable bythe formation of precipitates) and, on the other hand, leads toconsiderable extra costs because of the tin(IV) compounds which cannotbe used for coloring. Accordingly, various processes have beendeveloped, differing in particular in the means used to stabilize thegenerally sulfuric acid tin(II) sulfate solutions, for the electrolyticcoloring of aluminum.

Phenol-like compounds, such as phenol sulfonic acid, cresol sulfonicacid or sulfosalicylic acid, are by far the most commonly used (S. A.Pozzoli, F. Tegiacchi; Korros. Korrosionsschutz Alum., Veranst. Eur.Foed. Korros. Vortr. 88th 1976, 139-45; JP-A-78 13583, 78 18483, 77135841, 76 147436, 74 31614, 73 101331, 71 20568, 75 26066, 76 122637,54 097545, 56 081598; GB-C-1,482,390).

Polyhydric phenols such as, for example, the diphenols hydroquinone,pyrocatechol and resorcinol (JP-A-58 113391, 57 200221; FR-C-23 84 037)and the triphenols phloroglycinol (JP-A-58 113391), pyrogallol (S. A.Pozzoli, F. Tegiacchi; Korros. Korrosionsschutz Alum., Veranst. Eur.Foed. Korros., Vortr. 88th 1976, 139-45; JP-A-58 113391; 57 200221) andgallic acid (JP-A-53 13583) have also been described in this connection.

Another significant problem in electrolytic coloring is so-calledthrowing power (depth throwing) which is understood to be the ability ofa product to color anodized aluminum parts situated at differentdistances from the counter-electrode with a uniform color. Good throwingpower is particularly important when the aluminum parts used arecomplicated in shape (coloring of depressions), when the aluminum partsare very large and when, for economic reasons, several aluminum partshave to be simultaneously colored in a single coloring process andmedium color tones are to be obtained. In practice, therefore, highthrowing power is highly desirable because faulty production is avoidedand the optical quality of the colored aluminum parts is generallybetter. The process is made more economical by good throwing powerbecause several parts can be colored in a single operation.

Throwing power is not the same as uniformity and a clear distinction hasto be drawn between the two.

Although a coloring process may achieve high uniformity, it may stillhave poor throwing power; the reverse is also possible. In general,uniformity is only influenced by the chemical composition of theelectrolyte, while throwing power is also dependent upon electrical andgeometric parameters, such as for example the shape of the workpiece orits positioning and size.

DE-A-26 09 146 describes a process for coloring in tin electrolytes, inwhich throwing power is established through the particular circuit andvoltage arrangement.

According to DE-A-20 25 284, the use of tin(II) ions by themselvesreduces throwing power, particularly when tartaric acid or ammoniumtartrate is added to improve conductivity.

DE-C-24 28 635 describes the use of a combination of tin(II) salts andzinc salts with addition of sulfuric acid and boric acid and alsoaromatic carboxylic and sulfonic acids (sulfophthalic acid orsulfosalicylic acid). Good throwing power is said to be obtained, inparticular when the pH value is between 1 and 1.5. pH adjustment to1-1.5 is a basic prerequisite for good electrolytic coloring; the pHvalue cannot be a deciding factor for a particular improvement inthrowing power. There is no mention of whether the organic acids addedhave an effect on throwing power, nor is the throwing power achievedquantitatively described.

DE-C-32 46 704 describes a process for electrolytic coloring in whichgood throwing power is guaranteed by the use of special geometry in thecoloring bath. In addition, cresol and phenol sulfonic acid, organicsubstances, such as dextrin and/or thiourea and/or gelatine, are said toguarantee uniform coloring.

In addition, European patent application EP-A-354 365 describes aprocess for the electrolytic coloring of anodized aluminum surfacesusing metal salts, in which the antioxidants correspond to one ofgeneral formulae I to IV: ##STR1## in which R¹ and R² representhydrogen, alkyl, aryl, alkylaryl, alkylaryl sulfonic acid containing 1to 22 carbon atoms and alkali metal salts thereof and R³ represents oneor more hydrogen and/or alkyl, aryl, alkylaryl radicals containing 1 to22 carbon atoms, at least one of the substituents R¹, R² and R³ notbeing hydrogen, are used together with the throw improvers p-toluenesulfonic acid and/or naphthalene sulfonic acid. However, the throwimprovers mentioned in this document lead during electrolysis tofoul-smelling decomposition products.

In addition, German patent application P 40 34 304.9 describes a processfor the electrolytic coloring of anodized aluminum surfaces with metalsalts, in which a synergistic mixture of antioxidants corresponding toone of general formulae I to IV and throw improvers corresponding togeneral formula V: ##STR2## in which WR¹ to R⁵ represent hydrogen,hydroxyl, carboxyl and/or sulfonic acid groups, is used.

Besides antioxidants and throw improvers, however, the most importantconstituent of the coloring baths is tin(II) sulfate. This saltaccumulates as a finely crystalline substance during its production.This involves major problems from the applicational point of view. Onthe one hand, the powder-form product emits dust and is also difficultto dispense. On the other hand, oxidation products are formed in theevent of prolonged storage. Besides the solid, concentrated aqueoussolutions of tin(II) sulfate can also be obtained. Unfortunately, thesesolutions have the disadvantage of a small content of active substanceper unit volume and, during their replenishment, the coloring baths arein danger of overflowing through the introduction of water.

DESCRIPTION OF THE INVENTION object of the Invention

Now, the problem addressed by the present invention was to provide anapplicationally advantageous form for tin(II) sulfate for use in atin(II)-containing sulfuric acid coloring bath for the a.c. coloring ofanodized aluminum surfaces which would overcome the problems known fromthe prior art, such as guaranteeing lasting stability of the coloringbath, avoiding the oxidation of Sn(II) and, at the same time,guaranteeing good throwing power, in conjunction with easy dispensing ofa storable tin(II) sulfate.

SUMMARY OF THE INVENTION

The problems stated above are solved by manufactured tin(II) sulfategranules for the electrolytic coloring of anodized aluminum surfaceswith metal salts which are characterized in that they have a length of0.1 to 10 mm.

Although granules and/or granulation are referred to hereinafter, theinvention encompasses not only the irregularly shaped granules, but alsomicrospheres, i.e. pellets, produced by shaping of the moist material indrums or on inclined rotating pans as well as cylindrical, rectangularsolid or other particles of geometrically defined shape.

DESCRIPTION OF PREFERRED EMBODIMENT

The quantity of water to be added during granulation is, in particular,between 0.01 and 8% by weight and preferably between 1.0 and 2.5% byweight, based on the overall composition of the granules, to ensure thatthe particles adhere to one another. The moisture content of the tin(II)sulfate powder used for granulation has to be taken into considerationin this regard. In addition to the tin(II) sulfate, other ingredients ofthe granules, which will be discussed hereinafter, may also have to betaken into consideration in this regard. Accordingly, the final granulespreferably contain 0.01 to 8% by weight and, more preferably, 1.0 to2.5% by weight, of water.

Depending on the mask used in the granulating machine, the granulesobtained are, for example, cylindrical or rectangular solid in shape andare cut to lengths of 0.1 to 10 mm. In one preferred embodiment of theinvention, cylinders or rectangular solids with a height or edge lengthof 0.1 to 10 mm and, more particularly, 2 to 8 mm and a diameter orwidth of 0.8 to 2 mm and, more particularly, 0.9 to 1.5 mm areparticularly preferred. Further aftertreatments, for example rounding ofthe edges to produce beads, may of course also be carried out within thescope of the present invention.

The present invention also relates to manufactured tin(II) sulfategranules as defined above additionally containing antioxidants known perse, throw improvers and/or other heavy metal salts. The advantage ofintroducing these compounds into the granules is that the desiredconstituents can always be subsequently introduced during the coloringprocess in an exact ratio determined in advance. Accordingly, in onepreferred embodiment, manufactured tin(II) sulfate granules contain:

29.99 to 99.99% by weight of tin(II) sulfate,

0 to 10% by weight of antioxidants,

0 to 50% by weight of throw improvers,

0 to 30% by weight of heavy metal salts and

0.01 to 8% by weight of water,

the percentage contents of the above-mentioned constituents of thegranules adding up to 100% by weight.

In another preferred embodiment of the invention, the granules contain:

80 to 95% by weight and preferably 85 to 89% by weight of tin(II)sulfate,

0.5 to 2% by weight and preferably 1% by weight of antioxidants,

2 to 14% by weight and preferably 9 to 11% by weight of throw improversand

0.5 to 3.5% by weight and preferably 1.0 to 2.5% by weight of water,

the percentage contents of the above-mentioned constituents of thegranules adding up to 100% by weight.

In another preferred embodiment of the invention, the antioxidants areselected from at least one compound corresponding to one of generalformulae I to IV: ##STR3## in which R¹ and R² represent hydrogen, alkyl,aryl, alkylaryl, alkylaryl sulfonic acid, alkyl sulfonic acid containing1 to 22 carbon atoms and alkali metal salts thereof and R³ representsone or more hydrogen and/or alkyl, aryl, alkylaryl radicals containing 1to 22 carbon atoms, at least one of the substituents R¹, R² and R³ notbeing hydrogen.

According to the invention, throw improvers selected from at least onecompound corresponding to general formula V: ##STR4## in which R¹ to R⁵represent hydrogen, hydroxyl, carboxyl and/or sulfonic acid groups, mayalso be used in the manufactured tin(II) granules.

A major advantage of the manufactured tin(II) sulfate granules accordingto the invention lies in the use of oxidation-stable water-soluble throwimprovers. It is precisely in the event of prolonged operating timesthat the p-toluenesulfonic acid known from the prior art emitsfoul-smelling vapors through oxidation of the methyl group, and makesprolonged use of the coloring bath intolerable. According to theinvention, therefore, it is important to introduce oxidation-stablefunctional groups, such as carboxyl, hydroxyl and/or sulfonic acidgroups, into the throw improver where it is present. In addition, thefunctional groups mentioned guarantee the necessary solubility in waterin contrast to other throw improvers widely used in the prior art.

According to the invention, 2-tert-butyl-1,4-dihydroxybenzene(tert-butyl hydroquinone), methyl hydroquinone, trimethyl hydroquinone,4-hydroxy-2,7-naphthalene disulfonic acid and/or p-hydroxyanisole arepreferably used as antioxidants corresponding to general formulae I toIV.

According to the invention, particularly suitable throw improverscorresponding to general formula V are 5-sulfosalicylic acid,4-sulfophthalic acid, 2-sulfobenzoic acid, benzoic acid and/orbenzenehexacarboxylic acid.

According to the invention, the tin(II) sulfate granules may alsocontain other heavy metal salts, preferably the salts and/or oxides ofnickel, cobalt, copper, silver, gold and/or manganese which are solublein sulfuric acid. If these heavy metals are used in the form of salts,the corresponding sulfates or nitrates arc particularly appropriate. Ofthe heavy metal ions mentioned, nickel and copper arc preferred for thepurposes of the invention. The addition of nickel ions intensifies thecoloring effect, i.e., a larger quantity of tin is deposited in the sameunit of time. The addition of copper ions provides the typical tincolors with an optionally desirable red tone.

So far as the quantities of heavy metal ions used are concerned, it maybe said that the sum total of the heavy metal ions, including tin, ispreferably between 3 and 20 g/l and more preferably between 7 and 16 g/l

For example, an electrolyte of the type in question contains 4 g/l ofSn(II) ions and 6 g/l of Ni(II) ions, both in the form of dissolvedsulfate salts. An electrolyte such as this has the same coloringproperties as an electrolyte containing only 10 g/l Sn(H). An advantagelies in the lower sensitivity of the electrolyte to oxidation throughthe smaller quantity of Sn(II).

The tin(II) sulfate granules manufactured in accordance with theinvention may be obtained by thorough mixing of the constituentsmentioned above and subsequent granulation, optionally with dissipationof the heat generated during granulation. The granulation/pelletingoperation should preferably be carried out at temperatures in the rangefrom 10° to 70° C. In one particularly preferred embodiment, granulationis carried out at temperatures in the range from 40° to 60° C., in orderlargely to prevent oxidation during granulation.

The present invention also relates to the use of the manufacturedtin(II) sulfate granules for the electrolytic coloring of aluminumsurfaces with metal salts. A major advantage of the granules is thatthey are easy to add to the sulfuric acid coloring baths in measuredquantities. By addition of the granules, all the important constituentsfor the effective electrolytic coloring of anodized aluminum surfacesare simultaneously added to the dilute sulfuric acid in optimal quantityratios and in a readily soluble form. The quantity of granules per litercoloring bath is determined by the desired bath concentrations. However,if the tin(II) sulfate granules do not contain any or all of theadditional additives mentioned above, such as antioxidants, throwimprovers and/or heavy metal salts, it is of course possible inaccordance with the invention to introduce such additives into thecoloring bath in addition to the granules.

Coloring is normally carried out with a tin(II) sulfate solutioncontaining approximately 3 to 20 g and preferably 7 to 16 g of tin perliter. The coloring process is preferably carried out at a pH value of0.1 to 2.0 and, more particularly, at a pH value of 0.35 to 0.5,corresponding to 16 to 22 g sulfuric acid per liter, at a temperature inthe range from about 14° to 30° C. The a.c. voltage or the a.c. voltage(50 to 60 Hz) superimposed on direct current is preferably adjusted to avalue of 10 to 25 V and, more preferably, to a value of 15 to 18 V withan optimum of the order of 17 V.

In the context of the invention, the expression "alternating currentsuperimposed on direct current" is the same as direct currentsuperimposed on alternating current. The figure shown is always theterminal voltage. Coloring begins at a resulting current density ofgenerally about 1 A/dm² which then falls to a constant value of 0.2 to0.5 A/dm². The color tones obtained, which can vary from champagnethrough various bronze tones to black, differ according to the voltage,the metal concentration in the coloring bath and the immersion times.

The manufactured tin(II) sulfate granules are further distinguished byvery good solubility in dilute sulfuric acid, by the absence of dust andby excellent stability in storage.

EXAMPLES A) Production of pelleted tin(II) sulfate granules

The formulation ingredients (see Table) of Examples 1 to 7 were premixedfor 30 s in 2 to 5 kg batches in a 5 liter plowshare mixer (for examplea Lodige mixer). Water was then added (30 s), the aftermixing time being60 s. The premix was then pelleted with no further aftertreatment. Nocoarse particles occurred while fine particles and debris were removedby sieving. The product is ready to use after sieving. The suitablecirculation of cooling water and/or a suitable temperature of thecooling water ensured that the product temperature did not exceed 45° C.during the pelleting process. The particular feature of the pelletingpress used (Schluter type PP 127-3.0) is its thermostatically cooledrunner. The machine was operated at an r.p.m. setting of about 75%. Thecylindrical pellets obtained had a length of 2 to 5 mm and a diameter ofabout 1 mm.

B) Testing of the coloring bath properties

The pellets according to Examples 1 to 7 were added to the sulfuric acid(20 g/l) in such a quantity that an Sn(II) concentration of 10 g/l wasobtained.

Sample plates of the DIN material Al 99.5 (No. 3.0255) wereconventionally pretreated (degreased, pickled, descaled) and anodizedfor 60 minutes by the DC

                  TABLE                                                           ______________________________________                                        Example     1      2      3    4    5    6    7                               ______________________________________                                        Composition of the                                                            granules (% by                                                                weight):                                                                      Tin(II) sulfate                                                                           87.4   82.6   46.0 67.6 84.2 99.0 97.4                            t-Butyl hydro-                                                                            1.0    1.8    13.1 --   0.9  --   1.1                             quinone                                                                       Sulfosalicylic acid                                                                       9.6    14.3   39.4 29.9 --   --   --                              Methyl hydro-                                                                             --     --     --   1.1  --   --   --                              quinone                                                                       Benzenehexa-                                                                              --     --     --   --   14.0 --   --                              carboxylic acid                                                               Water       2.0    1.3    1.5  1.4  0.9  1.0  1.5                             Concentration used                                                                        20.5   21.9   39.34                                                                              26.8 21.5 18.1 18.1                            for 10 g/l Sn(II):                                                            Composition of the                                                            coloring bath (g/l):                                                          Sulfuric acid                                                                             20.0   20.0   20.0 20.0 20.0 20.0 20.0                            Tin(II)     10.0   10.0   10.0 10.0 10.0 10.0 10.0                            t-Butyl hydro-                                                                            0.2    0.4    5.15 --   0.2  --   0.2                             quinone                                                                       Sulfosalicylic acid                                                                       2.0    3.1    15.5 8.0  --   --   --                              Methyl hydro-                                                                             --     --     --   0.3  --   --                                   quinone                                                                       Benzenehexa-                                                                              --     --     --   --   3.0  --   --                              carboxylic acid                                                               Bath stability                                                                            1      1      1    1    1    5    1                               Coloring result                                                                           1      1      1    1    1    5    5                               15 V, 5 mins.                                                                 ______________________________________                                    

process (=direct current/sulfuric acid process, 200 g/l sulfuric acid,10 g/l Al(III), throughput of air, 1.5 A/dm², 18° C.). A layerapproximately 20 μm thick was built up. The plates thus pretreated werethen colored in the prepared coloring baths for 15 minutes using 15 Valternating current (50 Hz).

The coloring result and bath stability were compared with color finishesof electrolytes obtained by successive addition of the individual activesubstances to the dilute sulfuric acid. Scoring: 1=very good,6=inadequate. There were no differences in Examples 1 to 5. Althoughgood granules were obtained in Examples 6 and 7, they did not producethe required results in regard to bath stability and coloring due to theabsence of additives. In these cases, the corresponding additives, i.e.,antioxidant and throw improver, may be separately added to the coloringbath if desired.

The invention claimed is:
 1. Tin(II) sulfate-contained granules for theelectrolytic coloring of anodized aluminum surfaces with metal salts,which are manufactured by agglomerating moist solid tin(II) sulfate indrums or on inclined rotating pans and which have a length of 0.1 to 10mm.
 2. Tin(II) sulfate-containing granules as claimed in claim 1, whichcontain 1.0 to 2.5% by weight of water, based on the overall compositionof the granules.
 3. Tin(II) sulfate-containing granules as claimed inclaim 2, which are in the form of cylinders or rectangular solids with aheight or edge length of 2 to 8 mm and a diameter or width of 0.9 to 1.5mm.
 4. Tin(II) sulfate-containing granules as claimed in claim 3, whichcontain antioxidants, throw improvers, other heavy metal salts, or anytwo or more thereof.
 5. Tin(II) sulfate-containing granules as claimedin claim 4, which contain:29.99 to 99.99% by weight of tin(II) sulfate,up to 10% by weight of antioxidants, up to 50% by weight of throwimprovers, up to 30% by weight of heavy metal salts and 0.01 to 8% byweight of water,the percentage contents of the above-mentionedconstituents of the granules adding up to 100% by weight.
 6. Tin(II)sulfate-containing granules as claimed in claim 5, which contain:85 to89% by weight of tin(II) sulfate, 1% by weight of antioxidants, 9 to 11%by weight of throw improvers and 1.0 to 2.5% by weight of water,thepercentage contents of the above-mentioned constituents of the granulesadding up to 100% by weight.
 7. Tin(II) sulfate-containing granules asclaimed in claim 6, which contain at least one of:a) antioxidantsselected from at least one compound corresponding to one of generalformulae I to IV: ##STR5## in which R¹ and R² represent hydrogen, alkyl,aryl, alkylaryl, alkylaryl sulfonic acid, alkyl sulfonic acid containing1 to 22 carbon atoms and alkali metal salts thereof and R³ representsone or more hydrogen or alkyl, aryl, or alkylaryl radicals containing 1to 22 carbon atoms, at least one of the substituents R¹, R² and R³ notbeing hydrogen, and b) throw improvers selected from at least onecompound corresponding to general formula V: ##STR6## in which R¹ to R⁵represent hydrogen, hydroxyl, carboxyl or sulfonic acid groups. 8.Tin(II) sulfate-containing granules as claimed in claim 7, which containat least one compound selected from the group consisting of2-tert-butyl-1,4-dihydroxybenzene (tert-butyl hydroquinone), methylhydroquinone, trimethyl hydroquinone, 4-hydroxy-2,7-naphthalenedisulfonic acid and p-hydroxyanisole as antioxidant or at least onecompound selected from the group consisting of 5-sulfosalicylic acid,4-sulfophthalic acid, 2-sulfobenzoic acid, benzoic acid andbenzenehexacarboxylic acid as throw improver.
 9. Tin(II)sulfate-containing granules as claimed in claim 8, which contain othercoloring heavy metal pigments selected from salts and oxides of nickel,cobalt, copper, silver, gold and manganese that are soluble in sulfuricacid.
 10. Tin(II) sulfate-containing granules as claimed in claim 1,which contain 0.01 to 8% by weight of water, based on the overallcomposition of the granules.
 11. Tin(II) sulfate-containing granules asclaimed in claim 1, which are in the form of cylinders or rectangularsolids with a height or edge length of 0.1 to 10 mm and a diameter orwidth of 0.8 to 2 mm.
 12. Tin(II) sulfate-containing granules as claimedin claim 1, which contain antioxidants, throw improvers, other heavymetal salts, or any two or more thereof.
 13. Tin(II) sulfate-containinggranules as claimed in claim 1, which contain:29.99 to 99.99% by weightof tin(II) sulfate, 0 to 10% by weight of antioxidants, 0 to 50% byweight of throw improvers, 0 to 30% by weight of heavy metal salts and0.01 to 8% by weight of water,the percentage contents of theabove-mentioned constituents of the granules adding up to 100% byweight.
 14. Tin(II) sulfate-containing granules as claimed in claim 13,which contain:80 to 95% by weight of tin(II) sulfate, 0.5 to 2% byweight of antioxidants, 2to 14% by weight of throw improvers and 0.5 to3.5% by weight of water.
 15. Tin(II) sulfate-containing granules asclaimed in claim 4, which contain at least one of:a) antioxidantsselected from at least one compound corresponding to one of generalformulae I to IV: ##STR7## in which R¹ and R² represent hydrogen, alkyl,aryl, alkylaryl, alkylaryl sulfonic acid, alkyl sulfonic acid containing1 to 22 carbon atoms and alkali metal salts thereof and R³ representsone or more hydrogen or alkyl, aryl, or alkylaryl radicals containing 1to 22 carbon atoms, at least one of the substituents R¹, R² and R³ notbeing hydrogen, and b) throw improvers selected from at least onecompound corresponding to general formula V: ##STR8## in which R¹ to R⁵represent hydrogen, hydroxyl, carboxyl or sulfonic acid groups. 16.Tin(II) sulfate-containing granules as claimed in claim 4, which containat least one compound selected from the group consisting of2-tert-butyl-1,4-dihydroxybenzene (tert-butyl hydroquinone ), methylhydroquinone, trimethyl hydroquinone, 4-hydroxy-2,7-naphthalenedisulfonic acid and p-hydroxyanisole as antioxidant or at least onecompound selected from the group consisting of 5-sulfosalicylic acid,4-sulfophthalic acid, 2-sulfobenzoic acid, benzoic acid andbenzenehexacarboxylic acid as throw improver.
 17. Tin(II)sulfate-containing granules as claimed in claim 4, which contain othercoloring heavy metal pigments selected from salts and oxides of nickel,cobalt, copper, silver, gold and manganese that are soluble in sulfuricacid.
 18. A process of electrolytically coloring anodized aluminum bytreatment with alternating current in an electrolyte solution containingheavy metal pigment, wherein the improvement comprises utilizing, as atleast part of the heavy metal pigment, tin(II) sulfate-containinggranules according to claim
 1. 19. A process as claimed in claim 18,wherein the electrolyte solution contains sulfuric acid and from 3 to 20g/l of tin(II) ions.
 20. A process as claimed in claim 19, wherein theelectrolyte solution has a pH in the range from 0.35 to 0.5 and containsfrom 16 to 22 g/l of sulfuric acid, and the process is performed at 14°to 30° C. with a 50 to 60 Hz a.c. voltage, or a.c. imposed on directvoltage, of 15 to 18 V.